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Ed DunhamAdministrator
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)


Reged: Sun
Posts: 2565
Loc: Melbourne, FL
Eye of the Beholder
      Thu Jun 21 2012 01:54 PM

Chris has attained Hurricane status - the first hurricane of the season - which is remarkable given that the system is (and has been) over SSTs of 21C/70F. A temperature increase within the eye (as indicated by earlier FSU model outputs) seems to have provided the extra convective boost and subsequent wind increase to hurricane strength. The upgrade to hurricane intensity should be rather short-lived as the increasing forward motion to the north will take Chris into rapidly cooling SSTs.

From the NHC Glossary: "EYE: The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud."

Note the use of the term 'tropical cyclone' rather than 'hurricane'. Not every lower category hurricane has a well defined visible eye and not every well defined visible eye means that the system is a hurricane. Tropical Depressions have often been noted as having 'eye-like' features, and strong Tropical Storms have often had an eye feature. A recent example of this was Tropical Storm Cindy in the 2011 season which developed an eye over the north Atlantic (38N 49W) with 50 knot sustained winds:


TROPICAL STORM CINDY DISCUSSION NUMBER 3
NWS NATIONAL HURRICANE CENTER MIAMI FL AL032011
500 AM AST THU JUL 21 2011

CINDY HAS DEVELOPED A DOUGHNUT HOLE IN THE INNER CORE CONVECTION.
WHILE I AM NOT READY TO CALL THIS AN EYE FEATURE...IT DOES SUPPORT
INCREASING THE WIND SPEEDS. ALTHOUGH SATELLITE INTENSITY ESTIMATES
ONLY SUPPORT 35 KT...GRADIENT WIND COMPUTATIONS USING THE CENTRAL
PRESSURE AND NEARBY SHIP AND BUOY REPORTS OF 1022-1023 MB IN THE
SOUTHEAST QUADRANT SUPPORT SURFACE WINDS OF ABOUT 55 KT. AS A
RESULT...THE INITIAL INTENSITY IS ADJUSTED UPWARD TO 50 KT FOR THIS
ADVISORY...WHICH COULD BE A LITTLE CONSERVATIVE.
.
SOME SLIGHT STRENGTHENING IS POSSIBLE IN THE NEAR TERM AS CINDY
REMAINS OVER SSTS OF AT LEAST 24C.
.
FORECASTER STEWART

Given that Cindy attained 50 knot winds as a well developed system over SSTs of 24C, it is quite unusual for Chris to attain 65 knots over SSTs of 21C. Downward transport of strong winds through the stable marine layer in the north Atlantic is quite difficult at 24C let alone 21C. While hurricanes do form in the northern waters, they usually do so later in the season when the SSTs have warmed considerably.

What is the earliest (in the season) known tropical cyclone to attain hurricane strength above 40N Latitude in the Atlantic basin? Answer: Hurricane Chris on 6/21/2012. How many other tropical cyclones have done this prior to July? Answer: None. The only other early-season storm that came close was Hurricane #3 at 38.4N 65.3W on 6/19/1959. Considering that Chris is a record breaking event, the NHC did a good job of waiting for the data to firmly support the upgrade to hurricane status.
ED

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* Eye of the Beholder Ed DunhamAdministrator Thu Jun 21 2012 01:54 PM

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